1. Field of the Invention
The present invention relates generally to the field of power switching systems. More specifically, the present invention relates to controllers for electric power schemes, such as those utilizing transfer schemes and automatic power transfer switches.
2. Description of the Related Art
Three-phase transformers are ubiquitous in power transmission and distribution systems. They typically consist of one or more magnetic cores with primary and secondary coil windings. Their purpose in the electrical system is to change, or transform, the voltage amplitude from one value to another. In general, the incoming side is referred to as the transformer primary whereas the outgoing side is referred to as the secondary. There are many permutations of transformer construction and winding for a multitude of applications.
Where power continuity is vital, various types of establishments have a backup power source to account for situations in which the preferred power source is unavailable. In these configurations control systems typically are designed for unattended monitoring and decision making to ensure electrical loads are automatically powered from the best available source. For example, if the preferred source fails, an automatic power switching system is generally responsible to activate the alternate source and transfer the load in an appropriate manner. Once the preferred source becomes restored, the automatic power switching system can autonomously coordinate retransfer.
Typically, the preferred source is provided by a utility company that delivers power through a transformer that converts transmission (or sub-transmission) and distribution voltage levels to utilization levels. The alternate, or backup, source is typically a standby generator which does not require a transformer because its output voltage is already at the utilization level.
FIG. 1 is an illustration of power system deployment showing a series of automatic transfer switches 110, 116, 120 for selectively coupling loads to preferred and alternate power sources. The preferred source includes a three-phase transformer 102 having a primary winding 104 and secondary winding 106. The secondary winding 106 feeds a preferred power source bus 108, which feeds the automatic transfer switches 110, 116, and 120. These automatic transfer switches 110, 116, and 120 selectively couple their respective loads 112, 118, and 122, to either the preferred source bus 108 or the alternate power source via an alternate power source bus 114.
Many power outages in three phase systems are preceded by single-phase faults whereby only one phase is lost on the transformer primary side. Some power outages are limited to single-phase faults entirely, for example, where a fallen tree takes down only one power phase. It has been both observed and reported that a single-phase fault on the primary side of certain installations containing a transformer having a grounded Wye-Wye configuration can lead to erratic behavior in automatic power switching equipment. In such a configuration, the load transfer controller generally detects the single-phase fault as an undervoltage or voltage unbalance condition, or combination thereof. The controller subsequently initiates transfer to the alternate source and awaits restoration of the preferred source. As a result, the transformer becomes unloaded. Through the effects of magnetic flux linkage and core magnetization, the secondary phase corresponding to the failed primary phase can develop a regenerated voltage, which may give the appearance that the source has been restored to validity. For similar reasons, voltage may be regenerated on the primary side as well. In a regenerated voltage condition, the co-existence of two properly energized phases, a fully magnetized core structure, and magnetic flux linkage to the de-energized phase windings induces the regenerated voltage with an appropriate phase angle relationship to the two good phases. The loading on this regenerated voltage phase, including transformer impedance, directly influences the magnitude of the regenerated voltage. Therefore, as impedance loading decreases, the regenerated voltage magnitude converges towards nominal terminal voltage.
In the worst-case scenario, the regenerated voltage rises to a level deemed acceptable to the parameter settings of the load transfer controller. FIG. 2 is a graph illustrating a single phase voltage reading of a three phase system 202 exhibiting a phase voltage failure and a regenerated voltage condition. The single phase voltage failure occurs at time T0, and is followed by a drop in single phase voltage reading to below the minimum phase voltage threshold 204 of the system as specified in the load transfer controller. Consequently, a retransfer sequence commences. Times T1, T2 and T3 indicate unloading of the preferred source by automatic transfer switches. Following this unloading, the energized phases (not shown) cause a regenerated voltage in the failed single phase 202 which bring the single phase voltage reading 202 above the phase voltage threshold 204 at time T4. After this time, an automatic transfer switch may erroneously determine that the preferred voltage source has been restored and therefore initiate retransfer. When loads retransfer back to the preferred source transformer at time T5, the resultant loading once again exposes an undervoltage or voltage unbalance condition leading to another transfer back to the alternate source. In such a scenario conditions exist for an indefinite cycling of this behavior until the preferred source becomes truly restored or repaired. Such continual cycling impedes continuity of electric power and is detrimental to the operation of critical loads. A method and system are needed to accurately detect this condition and cause the transfer scheme to react accordingly.